A recent post on measuring chair tenons suggested the need for digital calipers. Subsequent discussion of calipers with accuracy to 1/128 inch or less would be warranted.
Seems to me that unless one was looking for an excuse to buy a new tool that this is a bit excessive.
In fact if one were to amortise the cost of the caliper over the number of chairs that most of us are likely to make allowing for average age and productivity its a bit over the top.

A suggested alternative
Drill a mortise in two halves and with an an appropriate reamer make two halves of the mortise. Subsequently cut one of the halves to an appropriate length ( making sure you cut off the correct end)
As you turn the tenon this piece can be used to test fit.
For compulsive turners rubbing the inside of the test piece with a pencil can be used for fine tuning but with the tapered tenons its probably excessive.
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Best wishes,
Bill Howes